Optical waveguide coupler circuit device

ABSTRACT

The present invention relates to an optical waveguide coupler device. As shown in FIG.  1,  the optical waveguide coupler device  8  comprises two optical waveguide cores  2, 3  formed on the surface of a substrate  1.  The optical waveguide cores  2, 3  are covered by lower and upper cladding layers  6, 7  both formed on the substrate  1;  and the two optical waveguide cores  2, 3  are brought close to and in parallel with each other at two sites to form two directional couplers  4, 5  there. The cross-section and refractive index of each of the optical waveguide cores  2, 3  and the parameters of other elements of the circuit are optimized such that an optical signal is reliably routed by the circuit without undergoing a wavelength shift even when the circuit is exposed to the changes of ambient temperature.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a polymer-based optical waveguide circuit stable to the changes of ambient temperature which is profitably used as a wavelength division multiplexer or demultiplexer in, for example, fiber optic communication.

[0003] 2. Description of the Related Art

[0004] In a dense wavelength division multiplexing (DWDM) system applied to optical carriers belonging to S, C and L bands, the wavelength independent waveguide coupler has been used that is capable of routing optical carriers comprising 80-100 nm bands around 1550 nm into two channels at a specified split ratio, being unaffected with their wavelengths. The coupler that routes optical carriers at a split ratio of 1:1 is called a 3 dB coupler and has been used in multiple fiber optic communication systems (K. Jinguji et al., “Mach-Zehnder interferometer type optical waveguide coupler with wavelength-flattened coupling ratio,” Electron. Lett., 1990, Vol. 26, No. 17, pp. 1326-1327).

[0005] Such a wavelength independent waveguide coupler includes a Mach-Zehnder interferometer type optical circuit 17 as shown in FIG. 8. The circuit comprises two waveguide cores 13, 14 formed on the surface of a substrate 20 prepared from a quartz or silicon wafer.

[0006] The waveguide cores 13, 14 are covered with lower and upper cladding layers 18, 19 both of which are formed on the substrate 20. When the core and cladding layers are mainly composed of silicon dioxide (SiO₂), the resulting optical circuit is called a quartz waveguide. When they are composed of a polymer, the resulting optical circuit is called a polymeric waveguide.

[0007] The Mach-Zehnder interferometer type optical circuit 17 comprises two directional couplers 15, 16 which are obtained by bringing the two waveguide cores 13, 14 close to and in parallel with each other. The Mach-Zehnder interferometer type optical circuit 17 receives an optical signal having a band width of 80-100 nm around 1550 nm either from a terminal 13 a or 14 a connected respectively to the waveguide core 13 or 14 and splits the signal at a split ratio of 1:1 to deliver two outputs each having an intensity half that of the transmitted signal from terminals 13 b, 14 b, or the other terminals of the waveguide cores 13, 14.

[0008] An application of such a Mach-Zehnder interferometer type optical circuit includes an interleaver as shown in FIG. 2. This interleaver with two polymeric waveguides 9, 10 comprises circuits 11, 12: each of the circuits is equivalent to the Mach-Zehnder interferometer type optical circuit 17 shown in FIG. 8, and the two circuits are arranged to each other in a point symmetrical manner. If it is required to separate wavelengths having a bandwidth of 100 nm, it is only necessary for the interleaver shown in FIG. 2 to be configured such that a proper optical path difference is inserted between the two waveguides 9, 10. This simplifies the designing of the interleaver.

[0009] For example, according to the interleaver shown in FIG. 2, it is possible by designing the waveguides 9, 10 so as to produce a proper optical path difference, to receive optical signals λ1, . . . , λn each having a band width of 80-100 nm around 1550 nm from a terminal 9 a of a waveguide 9, separate those signals according to their wavelengths, route them alternately into two channels, and deliver them as separate outputs from the other terminals 9 b, 10 b of the waveguides 9, 10.

[0010] However, because the interleaver shown in FIG. 2 includes the waveguides made from a polymer, its optical characteristics are apt to vary in the presence of changes of the ambient temperature, and thus the temperature range under which it can normally operate is rather limited. The temperature coefficient of the refractive index of a polymer material used for the construction of the waveguides is ten or more times as high as that of quartz. Therefore, if the ambient temperature changes, the polymeric waveguides 9, 10 and cladding layers covering those waveguides will undergo a great change in their refractive indices; the parameters of the optical circuit including those waveguides and cladding layers will also shift from the designed ranges, and the performance of the optical circuit will depart from the designed level. Degraded performance of an optical circuit with polymeric waveguides as a result of the alteration of ambient temperature is mainly ascribed to the following two reasons. The first reason is: a phase difference which is produced as a result of properly chosen path length difference between the two waveguides 9, 10 is modified in the presence of a change of ambient temperature, which in turn brings about a change in the central wavelength of the affected optical carrier. The second reason is: an optical carrier having passed the Mach-Zehnder interferometer type optical circuits 17 exposed to a change of ambient temperature is modified such that its wavelength shifts to a shorter or longer one. For example, the refractive index of a polymeric waveguide has a temperature coefficient of about −(1.1-1.8)×10⁻⁴/K, and if such a polymeric waveguide is exposed to a temperature change of 40° C., a carrier having passed through the waveguide will undergo a shift of about 6.5 nm in its wavelength. Assume that an interleaver with polymeric Mach-Zehnder interferometer type optical circuits separates carriers at 0.8 nm intervals and is exposed to a temperature change of 40° C., and thus a carrier having passed the Mach-Zehnder interferometer type optical circuits suffers a shift of about 6.5 nm in its wavelength. Then, the carrier will be routed to a channel by eight channels shifted from the one to which it should be routed. According to an experiment, if such an interleaver as above is exposed to a temperature change of 40° C., a carrier having passed through the Mach-Zehnder interferometer type optical circuits 11, 12 is modified so much that its output changes by ±2% or more at the ends of its band width, and that it is impossible to maintain the crosstalk between adjacent carriers at −30 dB or lower.

[0011] The first problem will be solved by adjusting the physical parameters of waveguides 9, 10 such that they satisfy the following equation for a wavelength having a given bandwidth:

∂β₁₀ /∂T=∂β ₉ /∂T(L ₉ /L ₁₀)  (1)

[0012] where β₉ and β₁₀ represent the transmission constants of waveguides 9 and 10 for a mode of optical carriers, L₉ and L₁₀ the lengths of light path of the waveguides 9, 10 enclosed by the Mach-Zehnder interferometer type optical circuits 11, 12, and T temperature.

[0013] For solving the second problem, it is necessary to redesign the overall structure of the interleaver because its Mach-Zehnder interferometer type optical circuits 11, 12 are too complex in their structure. Similar problems to the above are also observed in certain types of quartz optical waveguide circuits. The refractive index of a quartz material has a positive temperature coefficient whose absolute value is smaller than that of a corresponding polymeric material. Therefore, a known method for preparing an optical waveguide circuit from quartz consists of covering a quartz waveguide core with a polymeric coat whose refractive index has a negative temperature coefficient sufficiently large to cancel the positivity of the temperature coefficient of the quartz waveguide core. However, generally a polymeric material has a refractive index whose temperature coefficient has too large a negative value to cancel the positivity of the temperature coefficient of a quartz material. Naturally, this method can not be applied for the polymeric optical waveguide circuit here concerned.

[0014] A known method for compensating for the thermal characteristics of a polymeric waveguide core is to employ a substrate made from a polymer having a high thermal expansion. To put it more specifically, this method consists of employing a polymeric substrate which has a thermal expansion sufficiently high to cancel the negative temperature coefficient of the refractive index of a polymeric waveguide core. However, a substrate made from quartz or silicon generally has a low thermal expansion, and thus as far as based on this method, it will not be possible to integrate optical waveguide circuits on a silicon substrate as in the conventional electronic technology where semiconductor devices are integrated on a silicon substrate.

SUMMARY OF THE INVENTION

[0015] Accordingly, the object of the present invention is to provide a Mach-Zehnder interferometer-based polymeric waveguide circuit unaffected with the change of ambient temperature which is obtainable by arranging waveguides on a silicon or quartz substrate using conventional IC technology, while maintaining the advantage of low production cost which is the most important impetus for the introduction of polymeric optical waveguide circuits.

[0016] To achieve the above object, a first aspect of the present invention is to provide an optical waveguide coupler circuit device comprising a substrate; a polymeric lower cladding layer formed on the substrate; at least two polymeric optical waveguides formed on the polymeric lower cladding layer; a polymeric upper cladding layer covering the optical waveguides; and plural directional couplers which are obtained by choosing any pair from the at least two optical waveguides, and bringing them close to each other at plural sites, wherein each of the optical waveguides having two ends, one end serving as an input terminal and the other as an output terminal; and the two paired optical waveguides are configured such that, if the difference between their effective optical paths spanning between arbitrarily chosen adjacent directional couplers is defined to be ΔL, the ΔL=0.6 to 0.8 μm.

[0017] A second aspect of the present invention is to provide an optical waveguide coupler circuit device comprising a substrate; a polymeric lower cladding layer formed on the substrate; at least two polymeric optical waveguides formed on the polymeric lower cladding layer; a polymeric upper cladding layer covering the optical waveguides; and plural directional couplers which are obtained by choosing any pair from the at least two optical waveguides, bringing them close to each other at plural sites, wherein the two paired optical waveguides are configured such that, if the difference between their effective optical paths spanning between arbitrarily chosen adjacent directional couplers is defined to be ΔL, the ΔL=0.6 to 0.8 μm; and each of the plural directional couplers comprises the parallel sections of two optical waveguides.

[0018] Another object of this invention is to provide the optical waveguide coupler circuit device wherein the polymeric optical waveguide is made from a polymer having a refractive index of 1.5182 to 1.5667.

[0019] Another object of this invention is to provide the optical waveguide coupler circuit device wherein the polymeric lower cladding layer is made from a polymer having a refractive index of 1.5136 to 1.5620.

[0020] A further object of this invention is to provide the optical waveguide coupler circuit device wherein the polymeric upper cladding layer is made from a polymer having a refractive index of 1.5136 to 1.5620.

[0021] A further object of this invention is to provide the optical waveguide coupler circuit device wherein the length of the two optical waveguides of one directional coupler is chosen to be 0.031 to 0.072 mm while the length of the two optical waveguides of the other directional coupler is chosen to be 0.982 to 1.741 mm.

[0022] A further object of this invention is to provide the optical waveguide coupler circuit device wherein the gap between two parallel running waveguides is chosen to be 4.1 to 6.4 μm for both directional couplers.

[0023] A further object of this invention is to provide the optical waveguide coupler circuit device wherein each of the optical waveguides is configured to have an oblong cross-section having a width w and a thickness t.

[0024] A further object of this invention is to provide the optical waveguide coupler circuit device wherein each of the optical waveguides is configured to have a square cross-section.

[0025] A further object hereof is to provide the optical waveguide coupler circuit device wherein each of the optical waveguides is configured to have a square cross-section with a side of 6 to 8 μm.

[0026] A further object hereof is to provide the optical waveguide coupler circuit device wherein the substrate is made of a quartz plate.

[0027] A further object hereof is to provide the optical waveguide coupler circuit device wherein the substrate is made of a silicon plate.

[0028] A further object hereof is to provide the optical waveguide coupler circuit device wherein the substrate is made of a polyimide resin plate. Other objects and advantages will become apparent as this disclosure proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 shows a flat and lateral view of a polymeric optical waveguide coupler device representing an embodiment of this invention.

[0030]FIG. 2 shows a flat view of an interleaver as an application of the polymeric optical waveguide circuit.

[0031]FIG. 3 shows a graph representing the split ratio of the embodiment shown in FIG. 1 to an optical signal when the wavelength of the optical signal is varied and the ambient temperature is kept at 20° C.

[0032]FIG. 4 shows a graph representing the split ratio of the embodiment shown in FIG. 1 to an optical signal when the wavelength of the optical signal is varied and the ambient temperature is kept at 0° C.

[0033]FIG. 5 shows a graph representing the split ratio of the embodiment shown in FIG. 1 to an optical signal when the wavelength of the optical signal is varied and the ambient temperature is kept at 10° C.

[0034]FIG. 6 shows a graph representing the split ratio of the embodiment shown in FIG. 1 to an optical signal when the wavelength of the optical signal is varied and the ambient temperature is kept at 30° C.

[0035]FIG. 7 shows a graph representing the split ratio of the embodiment shown in FIG. 1 to an optical signal when the wavelength of the optical signal is varied and the ambient temperature is kept at 40° C.

[0036]FIG. 8 shows a top and lateral view of a Mach-Zehnder interferometer type optical waveguide circuit 17.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] The preferred embodiments of the present invention will be described below with reference to attached figures.

[0038] A polymeric optical waveguide coupler circuit devise 8 stable to the changes of ambient temperature representing an embodiment of this invention and shown in FIG. 1 comprises two optical waveguide cores 2, 3 formed on the surface of a substrate 1 prepared from a quartz plate, a silicon wafer or a polyimide resin plate and the like. The optical waveguide cores 2, 3 are covered by a lower cladding layer 6 and an upper cladding layer 7 both being formed on the substrate 1. The devise 8 further comprises two directional couplers 4, 5 each of which is obtained by bringing the two optical waveguides 2, 3 close to and in parallel with each other. With the optical circuit device 8, the cross-sections and refractive indices of the optical waveguide cores 2, 3, and other circuit parameters are optimized such that the performance of the device is maintained even when light passing through the device undergoes a shift in its wavelength, or the ambient temperature is changed.

[0039] How this is achieved will be described below.

[0040] Referring to FIG. 1, assume that light enters from a terminal 2 a of the optical waveguide core 2 into the optical circuit 8 of FIG. 1; and the light is split into two components with a split ratio of η which are then delivered as outputs from the other terminals 2 b, 3 b of the optical waveguides 2, 3. Then, the split ratio η expressed in power can be expressed by the following equation: $\begin{matrix} {{\eta = {\frac{\left| B_{out} \right|^{2}}{\left| A_{out} \middle| {}_{2}{+ \left| B_{out} \right|^{2}} \right.} = {a^{2} + b^{2} + {2\quad {ab}\quad {\cos \left( {{\beta\Delta}\quad L} \right)}}}}}{where}} & (2) \\ {a = {{\cos \left\lbrack {\frac{\pi}{2L_{c1}}\left( {L_{1} + L_{e1}} \right)} \right\rbrack}{\sin \left\lbrack {\frac{\pi}{2L_{c2}}\left( {L_{2} + L_{e2}} \right)} \right\rbrack}}} & (3) \\ {b = {{\sin \left\lbrack {\frac{\pi}{2L_{c1}}\left( {L_{1} + L_{e1}} \right)} \right\rbrack}{{\cos \left\lbrack {\frac{\pi}{2L_{c2}}\left( {L_{2} + L_{e2}} \right)} \right\rbrack}.}}} & (4) \end{matrix}$

[0041] In the above equations, A_(out) and B_(out) represent the amplitudes of the light waves delivered as outputs from the terminals 2 b, 3 b of the optical waveguide cores 2, 3; L_(c1) and L_(c2) the total coupling lengths of the directional couplers 4, 5; L₁ and L₂ the lengths of the parallel sections of the directional couplers 4,5; L_(e1), and L_(e2) the equivalent incremental lengths of the parallel sections of the directional couplers 4, 5; β the transmission constant of waveguides 9 and 10 for a mode of light waves; and ΔL the optical path difference between the two optical waveguide cores 2, 3.

[0042] L_(c1) and L_(c2), and L_(e1) and L_(e1) can be expressed as a function of the following parameters: λ, the wavelength of a light wave; w and t the width and thickness of the optical waveguide cores 2, 3; n_(g) the refractive index of the optical waveguide cores 2, 3; n_(c) the refractive index of the cladding layers 6, 7; and s₁ and s₂ gaps of the parallel sections of the directional couplers 4, 5. Therefore,

L _(ci) =L _(ci)(λ, w, t, n _(g) , n _(c) , s _(i)), i=1, 2  (5)

L _(ei) =L _(ei)(λ, w, t, n _(g) , n _(c) , s _(i)), i=1, 2  (6)

[0043] The transmission constant β can be expressed as a function of the following parameters: λ, the wavelength of a light wave; w and t the width and thickness of the optical waveguide cores 2, 3; n_(g) the refractive index of the optical waveguide cores 2, 3; and n_(c) the refractive index of the cladding layers 6, 7, as follows.

β=β(λ, w, t, n _(g) , n _(c))  (7)

[0044] The refractive index n_(g) of the optical waveguide cores 2, 3, and the refractive index n_(c) of the cladding layers 6, 7 can be expressed as a function of ambient temperature T and the wavelength λ of a light wave, as follows.

n _(g) =n _(g)(λ, T)  (8)

n _(c) =n _(c)(λ, T)  (9)

[0045] For a given light wave having a certain band width operating under a given temperature range, the width w and thickness t of the optical waveguide cores 2, 3; gaps s₁ and s₂ of the parallel sections of the directional couplers 4, 5; the refractive index n_(g) of the optical waveguide cores 2, 3; the refractive index n_(c) of the cladding layers 6, 7; the lengths L₁, L₂ of the parallel sections of the directional couplers 4,5; and the optical path difference ΔL between the two optical waveguide cores 2, 3 are properly chosen in order to satisfy the following simultaneous equations:

η(λ, T)=50%±δη  (10)

[0046] $\begin{matrix} {\left| {{\delta\eta}\left( {\lambda,\quad T} \right)} \right|_{\lambda} = \left| {\frac{\partial\eta}{\partial\lambda} \times {\delta\lambda}} \middle| {< \sigma_{\lambda}} \right.} & (11) \\ {\left| {{\delta\eta}\left( {\lambda,\quad T} \right)} \right|_{T} = \left| {\frac{\partial\eta}{\partial T} \times \delta \quad T} \middle| {< \sigma_{T}} \right.} & (12) \end{matrix}$

δη=|δη(λ, T)|_(λ)+|δη(λ, T)|_(T)  (13)

[0047] The resulting optical waveguide circuit will split a light wave at a specified split ratio independent of its wavelength, and being unaffected by the ambient temperature.

[0048] In the above calculation, σλ and σT are chosen so as to make δη≦1%.

[0049] The polymeric optical waveguide coupler circuit 8 stable to ambient temperature changes produced by the above method according to this invention has its parameters optimized as described below.

[0050] With regard to the optical circuit device as shown in FIG. 1, its substrate 1 is made from quartz, silicon or a polyimide resin and the like; the optical waveguides 2, 3 from a polymer having a refractive index n_(g) of 1.5182 to 1.5667; and the lower and upper cladding layers 6, 7 for covering the optical waveguides 2, 3 from another polymer having a refractive index n_(c) of 1.5136 to 1.5620. Each of the optical waveguides 2, 3 has a square cross-section with a side of 6 to 8 μm; the parallel section of the directional coupler 4 has a length L₁ of 0.031 to 0.072 or 0.982 to 1.741 mm; the parallel section of the directional coupler 5 has a length L₂ of 0.982 to 1.741 or 0.031 to 0.072 mm; the gap of the parallel waveguides in the directional coupler 4 has a size S₁ of 4.1 to 6.4 μm; the gap of the parallel waveguides in the directional coupler 5 has a size S₂ of 4.1 to 6.4 μm; and the optical path difference ΔL between the two optical waveguides 2, 3 is 0.6 to 0.8 μm.

[0051] Examples of the present invention will be described below with reference to attached figures.

EXAMPLE

[0052]FIG. 1 shows an optical waveguide coupler circuit device of this invention.

[0053] The polymeric optical waveguide coupler circuit device 8 stable to the changes of ambient temperature comprises two optical waveguide cores 2, 3 formed on the surface of a substrate 1 made of a quartz plate as shown in FIG. 1. The optical waveguide cores 2, 3 are made from a polymer material having a refractive index n_(g) of 1.5182 to 1.5667 as shown in the figure, and has a square cross-section with a side of 6 to 8 μm. The material constituting the lower and upper cladding layers 6, 7 covering the optical waveguide cores 2, 3 is a polymer having a refractive index n_(c) of 1.5136 to 1.5620.

[0054] The optical waveguide cores 2, 3 form two directional couplers 4, 5 by approaching to and running in parallel with each other at two sites. For the directional coupler 4, the length L₁ of the parallel section is 0.031 to 0.072 mm, and the gap S₁ between the parallel running waveguides is 4.1 to 6.4 μm. For the directional coupler 5, the corresponding length L and gap S₂ are 0.982 to 1.741 mm and 4.1 to 6.4 μm, respectively.

[0055] Needless to say, the values of L₁ and L₂ may be exchanged for with each other, that is, L₁ may take 0.982 to 1.741 mm and L₁ may take 0.031 to 0.072 mm.

[0056] The optical waveguide cores 2, 3 are configured such that the difference between their optical path lengths falls within 0.6 to 0.8 μm.

[0057] An exemplary method for fabricating the polymeric optical waveguide coupler circuit device 8 stable to the change of ambient temperature of this invention will be described below.

[0058] A quartz plate to serve as a substrate 1 is prepared. A solution of a polymer which will form a lower cladding layer is prepared by dissolving the same molecular amounts of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl in N,N-dimethylacetamide, and the solution is stirred at 25° C. for 24 hours in the presence of nitrogen. The resulting solution is applied by spin coating onto the substrate 1 to form a coat thereupon. The assembly is removed of the solvent, and subjected to a heating treatment, to form a polymeric lower cladding layer 6 having a thickness of about 20 mm on the substrate 1.

[0059] A solution of another polymer is prepared as follows: part of diamine that has been used for producing the polymer responsible for the formation of the lower cladding layer, that is, part of 2,2′-bis(trifluoromethyl)-4,4′-diamonobiphenyl is substituted for another diamine, that is, 4,4′-diaminodiphenyl ether, and the two diamines are added to 4,4′-(hexafluoroisopropylidene)diphthalic anhydride such that the summed molecular amounts of the two diamines are equal to the molecular amount of the latter. The resulting solution is treated as above, and is applied by spin coating onto the above assembly to form a coat on the coat to serve as the lower cladding layer. The assembly is removed of the solvent, and subjected to a heating treatment, to form a polymeric core layer with a thickness of about 8 μm which has a refractive index higher by about 0.25 to 0.45% than that of the lower cladding layer 6.

[0060] A specified optical waveguide pattern is formed via a photo-resist on the surface of the core layer. The core layer is pattern-etched via reactive ion etching in the presence of oxygen gas, to form optical waveguide cores 2, 3 having designed configurations. Then, the same polymer solution as is used for the formation of the lower cladding layer is applied by spin coating to the assembly. The resulting assembly is removed of the solvent and subjected to heating treatment. Thus, an upper cladding layer 7 is formed over the optical waveguide cores 2, 3 to embed the latter. The upper cladding layer 7 must have a refractive index higher than that of the waveguide cores, but needs not to have the same refractive index with that of the lower cladding layer 6.

[0061] The above example is related with a substrate 1 made of a quartz plate. The substrate 1 may be made of a silicon plate or a polyimide plate. The optical waveguide core may have an oblong cross-section with a width w and thickness t.

[0062] A performance test was conducted on the above exemplary polymeric optical waveguide coupler circuit device 8 stable to the changes of ambient temperature. The test results are as follows.

[0063] In an environment where the temperature was kept at 20° C., the polymeric optical waveguide coupler circuit 8 stable to the changes of ambient temperature as shown in FIG. 1 received seven light waves, that is, light waves with the wavelengths of 1490, 1510, 1530, 1550, 1570, 1590 and 1610 nm from a terminal 2 a of the optical waveguide core 2. The light waves were split during their passage through the circuit and delivered as outputs from the opposite terminals 2 b, 3 b of the optical waveguide cores 2, 3. For each light wave, the ratio of its power outputs from the two terminals (power split ratio) was determined, and the power split ratio was plotted as a function of wavelength as seen in FIG. 3. The power split ratio for any wavelengths enclosed in the band width of 120 nm is in the range of 50±0.69%.

[0064] Then, the ambient temperature was changed to 0, 10, 30, and 40° C., and the measurement as had been performed on the device kept at 20° C. was repeated. For each temperature, the power split ratio was plotted as a function of wavelength. The results are shown in FIGS. 4, 5, 6 and 7. As seen from the figures, the power split ratios are in the range of 50±0.67, 50±0.70, 50±0.69, and 50±0.68% respectively. They were hardly affected by the changes of ambient temperature.

[0065] As seen from FIGS. 3 and 7, the two wavelengths at the ends of the bandwidth, that is, 1490 and 1610 nm showed scarcely any notable variations in their power split ratios even when the ambient temperature was changed from 0° C. to 40° C., that is, the maximum variations actually observed were only 0.06 and 0.002% respectively. There was scarcely any band shift in the presence of a notable change of ambient temperature.

[0066] As seen from above, with the polymeric optical waveguide coupler circuit 8 of this invention stable to the changes of ambient temperature, optical signals falling within the bandwidth of 120 nm with 1550 nm at their central wavelength which are used in fiber optic communication based on dense wavelength division multiplexing are reliably routed because their wavelength being hardly modified during their passage through the circuit, even when the ambient temperature is changed from 0° C. to 40° C. Accordingly, the polymeric optical waveguide coupler circuit 8 of this invention stable to the changes of ambient temperature may serve, as one of its applications, as thermally stable couplers 11, 12 constituting a polymeric optical waveguide interleaver as shown in FIG. 2.

[0067] Needless to say, the use of the circuit of this invention is not limited to a polymeric optical waveguide interleaver. For example, the circuits of this invention may be connected to two 1×N channel optical waveguide splitters to produce a 2×2N channel optical waveguide splitter stable to the changes of ambient temperature.

[0068] According to this invention, the cross-sections and refractive indices of the waveguides and the parameters of other circuit elements constituting the polymeric optical waveguide coupler circuit device are optimized such that light passing through the device can reliably keep its wavelength unaffected by the changes of ambient temperature. The effect of the change of ambient temperature on the power split ratio of the device which would be otherwise manifest is obviously minimized.

[0069] As seen from above, with the polymeric optical waveguide coupler circuit device of this invention, optical signals falling within a bandwidth of 120 nm with 1550 nm at its central wavelength which are used in fiber optic communication based on dense wavelength division multiplexing are reliably routed even when the ambient temperature is changed from 0° C. to 40° C., without requiring a certain heat-insulating means.

[0070] Because the optical waveguide circuit device of this invention is optimized as described above, even wavelengths falling at the ends of the bandwidth are hardly affected by the change of ambient temperature, and there is no band shift either. Thus, it may be used, as one of its applications, as a thermally stable polymeric optical waveguide interleaver.

[0071] Because the method provided by the present invention optimizes the optical waveguide coupling circuit device using the same materials as used in the conventional optical waveguide device, it is not necessary to alter the conventional fabrication processes themselves. Thus, the method of this invention, maintaining the advantage of the conventional method of producing a polymeric optical waveguide device, that is, the low production cost, ensures the production of an optical waveguide circuit device stable to the changes of ambient temperature. 

What is claimed is:
 1. An optical waveguide coupler circuit device comprising: a substrate; a polymeric lower cladding layer formed on the substrate; at least two polymeric optical waveguides formed on the polymeric lower cladding layer; a polymeric upper cladding layer covering the optical waveguides; and plural directional couplers which are obtained by choosing any pair from the at least two optical waveguides, and bringing them close to each other at plural sites, wherein: each of the optical waveguides has two ends, one end serving as an input terminal and the other as an output terminal; and the two paired optical waveguides are configured such that, if the difference between their effective optical paths spanning between arbitrarily chosen adjacent directional couplers is defined to be ΔL, the ΔL=0.6 to 0.8 μm.
 2. An optical waveguide coupler circuit device comprising: a substrate; a polymeric lower cladding layer formed on the substrate; at least two polymeric optical waveguides formed on the polymeric lower cladding layer; a polymeric upper cladding layer covering the optical waveguides; and plural directional couplers which are obtained by choosing any pair from the at least two optical waveguides, and bringing them close to each other at plural sites, wherein: each of the optical waveguides has two ends, one end serving as an input terminal and the other as an output terminal; the two paired optical waveguides are configured such that, if the difference between their effective optical paths spanning between arbitrarily chosen adjacent directional couplers is defined to be ΔL, the ΔL=0.6 to 0.8 μm; and each of the plural directional couplers comprises a parallel section at which the two optical waveguides are disposed in parallel with each other.
 3. An optical waveguide coupler circuit device as described in claim 1 or 2 wherein the polymeric optical waveguide is made from a polymer having a refractive index of 1.5182 to 1.5667.
 4. An optical waveguide coupler circuit device as described in any one of claims 1 to 3 wherein the polymeric lower cladding layer is made from a polymer having a refractive index of 1.5136 to 1.5620.
 5. An optical waveguide coupler circuit device as described in any one of claims 1 to 4 wherein the polymeric upper cladding layer is made from a polymer having a refractive index of 1.5136 to 1.5620.
 6. An optical waveguide coupler circuit device as described in any one of claims 1 to 5 wherein the length of the two optical waveguides of one directional coupler is chosen to be 0.031 to 0.072 mm while the length of the two optical waveguides of the other directional coupler is chosen to be 0.982 to 1.741 mm.
 7. An optical waveguide coupler circuit device as described in any one of claims 1 to 6 wherein the gap between two parallel running waveguides is chosen to be 4.1 to 6.4 μm for both directional couplers.
 8. An optical waveguide coupler circuit device as described in any one of claims 1 to 7 wherein each of the optical waveguides is configured to have an oblong cross-section having a width w and a thickness t.
 9. An optical waveguide coupler circuit device as described in any one of claims 1 to 8 wherein each of the optical waveguides is configured to have a square cross-section.
 10. An optical waveguide coupler circuit device as described in any one of claims 1 to 9 wherein each of the optical waveguides is configured to have a square cross-section with a side of 6 to 8 μm.
 11. An optical waveguide coupler circuit device as described in any one of claims 1 to 10 wherein the substrate is made of a quartz plate.
 12. An optical waveguide coupler circuit device as described in any one of claims 1 to 10 wherein the substrate is made of a silicon plate.
 13. An optical waveguide coupler circuit device as described in any one of claims 1 to 10 wherein the substrate is made of a polyimide resin plate. 